Efficient Zirconocene-Coupling of Silicon-Substituted Diynes to Polymers and Macrocycles

The zirconocene-coupling of diynes with internal silicon substituents, MeC⋮CMe 2 SiArSiMe 2 C⋮CMe ( 1: Ar = 1,4-C 6 H 4; 2: Ar = 1,3-C 6 H 4; 3: Ar = 4,4‘-C 6 H 4 C 6 H 4 ), generates regiospecific polymers containing zirconacyclopentadiene in the main chain ( 5 − 7 ). These organometallic polymers hydrolyze cleanly to butadienediyl polymers of the type [Me 2 SiArSiMe 2 CH CMeCMe CH] n ( 11 − 13 ), and polymer 5 reacts with iodine to give the iodine-containing polymer [1,4-Me 2 SiC 6 H 4 SiMe 2 C(I) CMeCMe C(I)] n ( 14 ). The organometallic polymers undergo facile and high-yield degradations to macrocycles under mild conditions (refluxing tetrahydrofuran solution). The size and shape of the resulting macrocycles depend upon the nature of the diyne spacer group. Thus, polymers 5 and 7 containing parallel diyne units convert to the trimeric macrocycles [Me 2 SiArSiMe 2 C 4 Me 2 ZrCp 2 ] 3 ( 15: Ar = 1,4-C 6 H 4; 24: Ar = 4,4‘-C 6 H 4 C 6 H 4 ), while polymer 6 gives the dimeric macrocycle [1,3-Me 2 SiC 6 H 4 SiMe 2 C 4 Me 2 ZrCp 2 ] 2 ( 18 ). The dimeric macrocycle [Me 2 SiC 6 H 4 SiMe 2 C 6 H 4 SiMe 2 C 4 Me 2 ZrCp 2 ] 2 ( 20 ) was obtained directly from the zirconocene coupling of Me 2 Si[(1,4-C 6 H 4 )SiMe 2 (C⋮CMe)] 2 ( 4 ) by heating the reaction mixture to reflux. In a similar manner, the diyne Me 2 Si(C⋮CMe) 2 was converted in high yield to the hexameric macrocycle [Me 2 SiC 4 Me 2 ZrCp 2 ] 6 ( 22 ). The macrocycles 15, [1,4-Me 2 SiC 6 H 4 SiMe 2 C 4 Me 2 H 2 ] 3 ( 16 ), and 18 were characterized by single-crystal X-ray crystallography. Molecules of 15 adopt a nearly planar C 3 macrocyclic structure with a cavity described by an average transannular Si···Si distance of 13.2 Å, while the hydrolyzed macrocycle 16 has a chair conformation. This conformation change results from conversion of cis diene groups in the zirconacyclopendiene fragments to trans diene groups in 16 . The high-yield formation of macrocycles apparently results from the reversible nature of the alkyne-coupling reaction, which allows for a low-energy pathway to the smallest macrocycle possessing minimal ring strain.